(57) A process for preparing a solidified material containing a coal ash comprises a first step of mixing the coal ash with a calcium compound to obtain a mixture, a second step of molding the mixture to obtain a molded article, and a third step of subjecting the molded article obtained in the second step to a hydrothermal treatment at a temperature of 120°C or more under a high pressure. In the first step, 40 to 95 parts by weight of the coal ash is mixed with 60 to 5 parts by weight of the calcium compound, and the coal ash to be used has a bulk density of 0.8 g/cm³ or more, an average particle diameter of 5 to 40 µm and an aluminum content of 35% by weight or less in terms of Al₂O₃. By the use of a blowing agent or the like, it is also possible to obtain a porous light-weight solid. In consequence, there can be obtained the solid containing the coal ash and having a high mixing ratio of the coal ash, an excellent dimensional stability to water, a high strength and a decreased unevenness of the strength, and this kind of solid can be applied to many fields as building materials, construction materials, artificial aggregates and the like. Furthermore, a laminated solid having not only the above-mentioned characteristics but also weight reduction properties can also be obtained, and this solid can be suitably applied to building materials such as panels, blocks, bricks and boards for accoustic use, artificial light-weight aggregates and the like. Accordingly, the coal ash which is an industrial waste can be effectively utilized in large quantities, so that the conventional hard and troublesome problem of treating the coal ash can be solved or relieved.